1. Field of the Invention
The present invention relates to variable attenuators for optical transceivers such as laser rangefinders for ordnance control.
2. Description of the Prior Art
A laser rangefinder is an optical transceiver used for measuring the distance to a target. A laser rangefinder generally includes a laser transmitter for irradiating the target with pulses of polarized light, a photodetector for receiving the light reflected from the target, and a signal processor for measuring the time required for a light pulse to propagate to the target and back, thereby indicating the distance between the rangefinder and the target.
A desirable feature in a laser rangefinder is the ability to adjust the intensity of the laser beam transmitted to the target. For example, during training it is preferable to attenuate the beam so that personnel accidentally looking at it will not suffer eye damage. As another example, during combat it is advantageous to attenuate the laser beam to the minimum intensity required for reliable range measurements in order to minimize the detectability of the beam by the enemy. Since this minimum intensity depends on such variables as atmospheric conditions, target reflectivity and target distance, the ability to continuously adjust the attenuation over a wide range is desirable.
One known method of attenuating the beam transmitted by a laser transceiver is to insert in the beam transmission path an absorptive material or filter which allows only a small fraction of the incident light to be transmitted therethrough, the rest of the light energy being absorbed by the absorptive material and dissipated as heat. However, one disadvantage of this method is that the resulting heat can distort the filter so that it acts like a lens and disturbs the collimation of the transmitted beam. Another disadvantage of this method is that the absorptive filter imposes a fixed amount of attenuation with no provision for adjustment.
A known alternative to the preceding method is to position a partial reflector in the transmission path in place of the aforementioned absorptive filter, the reflector being designed to transmit only a small fraction of the incident light, the rest being reflected to where an absorptive material is positioned to absorb the reflected light. This alternative method overcomes the first shortcoming of the preceding method because the absorptive material is not in the path of the transmitted light. However, it still lacks any provision for adjusting the amount of attenuation produced by the reflector.